Project Summary The Word Health Organization (WHO) estimates that seasonal influenza causes about one billion infections, 3- 5 million cases of severe disease, and between 300,000 and 500,000 deaths around the word every year. Vaccination is the first line of defense against influenza, but the ever-changing nature of these viruses make vaccines ineffective after a single season or against pandemic strains. Hence, the appeal for more broadly protective influenza virus vaccines that result in protection against all or several influenza viruses and long- lasting immunity. After a typical influenza infection, both IgA and IgG responses are detected at the mucosal level with neutralizing activity against influenza. IgA responses are considered of great significance to prevent and/or control a myriad of genital, intestinal, and respiratory infections, including influenza. IgA, particularly secretory IgA (sIgA) in its multiple multimeric forms is typically more cross-reactive than IgG. IgA neutralizes pathogens without causing inflammation because of its inability to fix and activate the complement cascade. In contrast, IgG responses that bind but do not neutralize the virus (non-protective antibodies) have been linked to immune complex?mediated disease. Thus, a vaccine strategy aimed at improving mucosal IgA responses against influenza would be ideal to enhance and/or broaden protection against antigenic drifted variants or clades while minimizing the risks associated with suboptimal antibody-antigen match, disease enhancement or immunopathology. The influenza vaccines available to date have not been designed to specifically upregulate IgA responses and/or tilt the IgA/IgG balance towards more prominent IgA responses, particularly within the confinements of the airway. Currently approved or experimental live attenuated influenza virus vaccines are thought to stimulate mucosal and systemic IgA and IgG as well as T-cell mediated immunity because they more closely resemble a natural virus infection, but their relative influence on promoting IgA responses is not well understood. The IgA-inducing protein (IGIP), initially characterized in the bovine gastrointestinal associated lymphoid tissue (GALT), was shown to positively regulate IgA expression. We hypothesize that an ideal influenza vaccine would be one that efficiently stimulates IgA at mucosal surfaces to stop the virus from ever reaching the target cells. We further hypothesize that a live attenuated virus that produces IGIP (IGIP- LAIV) will upregulate mucosal IgA responses against influenza challenge. It is therefore the goal of this application to develop a vaccine strategy that will result in enhanced IgA responses against influenza after vaccination. To our knowledge, no influenza vaccine exists that specifically promotes IgA responses while retaining efficient stimulation of other humoral and T-cell protective responses. The IGIP-LAIV strategy proposed in this application is the first attempt to enhance protective IgA responses against influenza viruses.